The present invention relates to a solar concentrator, particularly a space solar concentrator, to form a solar panel.
Spacecraft typically carries solar cells as a primary energy source. The solar cells are positioned and oriented on the spacecraft so that they are exposed to solar radiation.
On body-stabilized spacecraft, solar cells are typically arranged in planar arrays and carried on solar wings which extend from opposite sides of a spacecraft body. Preferably, the solar wings rotate to keep them as orthogonal to the solar radiation as possible. Because the solar wings can be quite long in their deployed configuration, they are generally formed of a plurality of planar solar panels which are coupled together in an accordion arrangement (one-dimensional deployment) or in a paving arrangement (two-dimensional deployment) so that they can be collapsed to a smaller stowed configuration for spacecraft launch.
The number of solar cells that must be carried by a spacecraft is a function of the anticipated spacecraft power demand and the efficiency of the solar cells. Although high-efficiency solar cells reduce the number of cells required by a specific spacecraft, they are quite expensive. Because weight and weight-related costs also increase with the number of solar cells, there is a considerable incentive to reduce the quantity of solar cells that a spacecraft must carry.
Accordingly, efforts have been extended to concentrate solar radiation upon solar cells by using reflective surfaces that are positioned adjacent to solar panels and oriented to reflect additional radiation onto the cells. Solar radiation that would otherwise have passed by a solar wing is thus redirected to be incident upon the solar cells. Although a solar cell""s efficiency in conversion of this additional reflected radiation to useful energy is typically less than it is for the directly incident radiation, primarily due to increased cell temperature and slanted angle of incidence, solar concentration allows the number of spacecraft solar cells to be significantly reduced with consequent savings in spacecraft weight and cost. Both rigid and flexible reflectors have been proposed for solar radiation concentration with flexible reflectors generally having a weight advantage. An exemplary flexible reflector system is shown in U.S. Pat. Nos. 6,017,002 and 6,050,526. An exemplary rigid reflector system is shown in U.S. Pat. No. 5,520,747.
Although these reflector systems concentrate solar radiation,.their positioning adjacent to solar panel give rise to several drawbacks. The solar cell temperature increases and consequently the power conversion efficiency decreases. The pointing errors induces lack of flux uniformity on the cell panel and the power management is complicated, consequently decreasing the panel electric power collection.
In the case of deployable reflectors, the position of the reflectors and their deployment is not easily compatible with a two-dimensional deployment of the solar panels (paving-type panels) but only with a one-dimensional deployment thereof (accordion panels). Reflectors described in U.S. Pat. No. 5,520,747 present another pertinent drawback. The solar reflectors are stowed over the solar cell face of the solar panels. Accordingly, they block the use of the solar panels during any period (e.g., a transfer orbit) in which the solar panels are in a storage position that prevents reflector deployment. Moreover, the entire spacecraft power generation can be jeopardized in case of failure during reflector deployment.
Another type of concentration with reflectors consists in distributing small reflectors on the solar panel. Reflectors are laying in between solar cell rows, alternatively. It reduces or cancels several of the mentioned drawbacks. The present invention according to its second aspect is related to this kind of configuration. U.S. Pat. No. 6,188,012 and WO 00/79593 A1 are also describing some embodiments based on this geometric concept.
U.S. Pat. No. 6,188,012 applies only to a deployable concentrator. The deployment of the reflector is ensured thanks to several kinds of springs. After deployment, the spring is used to keep the reflective film under tension. The main drawback of such a device is the mechanical fatigue that occurs after a long time in space (with thermal cycling during each eclipse). For telecommunication spacecraft, the solar array must stay fully operational for 15 years in geostationary orbit. One eclipse per day occurs. More than 5,000 thermal cycles will result from more than 5,000 daily eclipses. If the reflector tension is progressively altered due to spring relaxation, the optical quality and the illumination uniformity will degrade. The effective concentration factor will vanish, with a significant loss for the spacecraft power generation. For that reason, after deployment, the reflective films need a fixer to ensure that no more mobility can produce the loss of tension. This patent is furthermore presenting deployment/storage concepts that are not fully valid. When the reflectors are stowed, their length looks smaller than when they are in deployed configuration. A realistic drawing would certainly depicts that, in the stowed configuration, the reflector film is partially shading the solar cells. In case of reflector deployment failure, the reflective films are shadowing the solar cells and the resulting power generation is vanishing. This is another drawback that one aspect of the present invention intends to avoid.
WO 00/79593 A1 is presenting a concept with self-deployable reflectors. They are clearly shadowing the cells in the stowed configuration. There is no blocking mechanism after deployment. During storage, the solar panels are conventionally mounted in stack with small space in between. The stowed reflectors are using this available space but, since no locking mechanisms are present in the stowed configuration, the reflectors of panel i are collapsed against reflectors from the next panel (i+1).
This configuration is doubtful since vibration (during transportation and launch, for instance) could generate scratches on the reflective films, altering the optical quality and later the effective solar concentration with a loss of power generation.
A solar concentrator according to a first aspect of the invention is defined in claim 1. It aims at providing a compact and robust structure, the rigidity of which is achieved by a combination of wedge-like reflectors and an honey comb panel, which also brings an enhanced cooling efficiency.
Further advantageous embodiments of this concept appear in the sub-claims.
With rigid reflectors (without deployment sequence), the geometric concentration is preferentially reduced to 1.6:1 in order to significantly reduce the reflector height (46% of the height of a reflector with concentration 2:1). The resulting solar array has a height which is still very close to the one of an array without concentration (no reflectors) and no unsafe deployment occurs, which greatly enhances the reliability of the concept.
A solar concentrator according to the first aspect of the present invention is composed of a rigid solar panel with rows of solar cells and reflectors (sawtooth shape) alternatively attached to the panel. The reflectors may be oriented at 30 degrees with respect to the perpendicular to the panel to reflect solar flux into cells with a concentration factor of 2:1. The reflector size depends on the solar cell size and concentration factor. When concentration factor is 2:1, the width is the same as the cell width. The length is the same as the panel element length.
According to a second aspect of the invention, the sawtooth (or wedge like) reflectors are deployable, and in the stowed position, the reflectors do not overlap the cell rows.
After deployment, reflectors collect and concentrate the solar flux to the cells. Before deployment of the reflectors, one of the preferred embodiment uses reflectors folded on the panel substrate to keep the folded geometry as compact as the one reached by a classic rigid panel without concentrator.
According to the first or second aspect of the invention, the reflectors may be made of thin film with metal deposited onto. In one of the embodiments, the film can be tight on a rigid light-weight frame with applied pre tension. In another embodiment, only half of the reflector is made of a film tight on a rigid frame. In another embodiment, the reflectors are made of rigid light-weight material like Carbon Fiber Reinforced Polymer (CFRP) or thin Nickel sheet. In another embodiment of the invention, the reflectors are made of thin film without rigid frame, bonded to the panel substrate at the edges or integrated in the panel structure. The film shape is produced by tension thanks to arms (possibly deployable) bonded to the panel and reaching the roof of the saw tooth reflector(s).
In the present invention, reflectors are replacing solar cell rows. The weight of GaAs solar cell is about 0.85 kg/m2. Including coverglass, connections and wires, the weight of a solar cell row is about 1.2 kg/m2. A thin film reflector is dramatically lighter. For instance, a 50 micron (2 mils) Kapton(copyright) film weights 71 g/m2 or a 10 micron Ni alloy shim weights 89 g/m2 only. Even including structural and mechanical parts for stowage locking, deployment, and final blocking, the weight of the solar panel is never increased by the addition of reflectors according to the invention. The reflective film can be made of other substrate than Kapton. For instance Mylar(copyright) and LaRC CP-1 films are good alternatives.
From the cost point of view, solar reflectors are less expensive than the equivalent solar cell area, which constitutes an additional improvement of the present invention.
Since body-stabilized spacecraft are equipped with a one-axis tracking capability, the pointing is relatively precise in the east-west plane (about xc2x12 degrees). No tracking is performed in the north-south plane. It results in a seasonal variation of the panel orientation with respect to the sun. About xc2x123.5 degrees variation occurs in the north-south axis. For that reason, concentrator are often linear, concentrating sunlight in the direction where tracking is performed. For that purpose, the present reflector rows are oriented along the north-south axis and concentrate solar flux in the tracking axis only. The trough reflector-type with a geometric concentration of 2:1 and reflectors oriented at 60 degrees with respect to solar panel reaches a collection efficiency loss of 10% when the off-pointing in the tracking axis is xc2x16.5 degrees. This never happens unless attitude control gets lost. Since no concentration is performed in the other axis, the seasonal variation has no significant influence on the solar flux collection, compared to solar panel with no concentration.
The use of solar reflectors integrated in the solar panel allows for a more versatile and modular design of the deployed solar panel, compared with the previous invention where reflectors are adjacent to solar panels (trough-type concentrators). Indeed, in the later case, the solar panel deployment will more easily happens only in a one-dimensional sequence, accordion-type. It results in a wide wingspan with alignment and control complexity. The present invention is still compatible with more complex deployment schemes like two-dimensional paving. The modularity is significantly improved compared to the previously mentioned inventions.
The thermal behaviour of solar panel is an important parameter.
The prior art trough-type concentrator (see for e.g. FIG. 2) increases the solar flux on the panel but there is no easy way to reject the additional heat. The cell temperature increases by 30-40 degrees resulting in an unwanted cell efficiency decrease. This is mainly due to the fact that the flux collection surface is increased by the reflectors but the cooling is still coming from the same area: the panel rear and front surfaces, which are facing the cold space environment.
In the present invention, reflectors are mounted on the panel and the solar flux is still concentrated by the same amount on the solar cell rows. However, the collection surface is not significantly enlarged. It remains almost identical to the non concentration panel surface where the cooling surface is the same as the sun irradiated surface. Only a small temperature increase is expected. The power conversion efficiency is better compared to trough concentrators.
With a one-axis solar tracking, 1 or 2 degrees off-pointing currently occurs. The solar flux distribution on the panel is perturbed. The distribution is no more uniform.
In a trough-type concentration panel, off-pointing will overexpose some cell rows and underexpose other rows. The photovoltaic cells perform the electric conversion. The produced electric current is directly related to the absorbed solar flux. Some cell rows will produce larger current than others. The serial connection of cells is not compatible with such a current variation. Unless large improvement of the power management is introduced, this non uniformity induces loss of electric power collection for the whole panel.
The present invention does not suffer from this drawback of uniformity lack. Since each pair of reflectors is acting on a single cell row, the offpointing induces a non uniform flux, distributed along the width of each cell and identical for each cell. The cell power conversion is identically affected on each cell and on each cell row. The induced electric current is still the same for each cell. The power collection by serial connection is no more affected. Power management is unchanged compared to non concentration panel and no additional loss is observed due to the off-pointing.
High Reflectivity of the reflectors results from the use of vacuum deposited aluminum (VDA) or preferentially over-protected silver coatings. Other coatings can be used as long as high solar reflectivity is produced. In the cell response spectrum, the average reflectivity of aluminum film at 60 degrees incidence with respect to the reflector normal is about 89%. Silver coating protected with SiO2 optimized thin layer, for instance, enhances the average reflectivity in the same condition to 97%. The over cost is easily compensated by the solar flux collection improvement.
The reflectors used in the present invention look like narrow tape. The width is about the same size as the cell width (xc2x140 mm). Film quality like micro-roughness or shape accuracy is more tolerant or easier to accommodate than the large reflectors used in a trough-type concentrator (typical width xcx9c2 m). This makes the design and manufacturing of the film and support easier. It also could contribute to reduce the weight of reflectors.